Recently, there has been growing interest in energy storage technologies. As energy storage technologies are extended to devices such as cellular phones, camcorders and notebook PC, and further to electric vehicles, demand for high energy density of batteries used as a source of power supply of such devices is increasing. Therefore, research and development of lithium secondary batteries, which most meet the demand, are actively being conducted.
Among secondary batteries currently used, a lithium secondary battery developed in the early 1990's comprises an anode made of carbon material capable of intercalating or disintercalating lithium ions, a cathode made of lithium-containing oxide, and a non-aqueous electrolyte solution obtained by dissolving a suitable amount of lithium salt in a mixed organic solvent.
The lithium secondary battery has an average discharge voltage of about 3.6 to 3.7V, which is advantageously higher than those of other batteries such as alkali batteries or nickel-cadmium batteries. In order to provide such a high operation voltage, an electrolyte composition electrochemically stable in a charging/discharging voltage range of 0 to 4.2 V is required. For this purpose, a mixed solvent in which a cyclic carbonate compound such as ethylene carbonate and propylene carbonate, and a linear carbonate compound such as dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate are suitably mixed to be used as a solvent of the electrolyte solution. A typical electrolyte solution uses lithium salt such as LiPF6, LiBF4 and LiClO4 as solutes, which acts as a source for supplying lithium ions in a battery and thus enables the lithium battery to operate.
During the initial charging process of the lithium secondary battery, lithium ions emitted from a cathode active material such as a lithium-metal oxide is transferred into an anode active material such as graphite and inserted between the layers of the anode active material, at which high reactive lithium reacts with the electrolyte solution and carbon present in the anode active material on the surface of the anode active material such as graphite to produce a compound such as Li2CO3, Li2O and LiOH. The produced compound forms a kind of a solid electrolyte interface (SEI) layer on the surface of the anode active material such as graphite.
The SEI layer functions as an ion tunnel, allowing only lithium ions to pass through. As an effect of such an ion tunnel, the SEI layer prevents the molecule of an organic solvent having large molecular weight, which is included in the electrolyte solution and transferred together with lithium ions, from being inserted between the layers of the anode active material to destroy the structure of the anode. As a result, direct contact of the electrolyte solution with the anode active material is prohibited to prevent the decomposition of the electrolyte solution and reversibly maintains the amount of lithium ions present in the electrolyte solution, thereby allowing a stable charging/discharging.
However, during the formation of the SEI layer, the carbonate-based organic solvent may decompose to generate a gas such as CO, CO2, CH4 and C2H6, which may cause the battery being charged to swell in thickness. Also, when a battery is left at a high temperature in a fully charged state, the SEI layer may be slowly broken down due to increased electrochemical energy and thermal energy over time, thereby causing continuous side reactions between the surface of the anode and the surrounding electrolyte solution and continuously generating gas. As a result, the inner pressure of the battery may be increased, thereby increasing the thickness of the battery to cause performance problems to electronics such as cellular phones and notebook computers equipped with the battery. Thus, the SEI layer has poor stability at a high temperature. In addition, in conventional lithium secondary batteries comprising a large amount of ethylene carbonate, the unstableness of the SEI layer may intensify the inner pressure increase problem of the battery. Furthermore, ethylene carbonate has a high freezing point (37 to 39° C.) and maintains a solid state at room temperature to have low ionic conductivity at a low temperature. Accordingly, a lithium battery using a non-aqueous solvent containing a large amount of ethylene carbonate has a poor conductivity at a low temperature.
In order to overcome such a problem, attempt has been made to vary the composition of a carbonate-based organic solvent or mix the solvent with a certain additive to change a SEI layer-forming reaction. However, it is known that the variation of a solvent composition or the addition of a certain compound may improve some performances of a battery, but may also deteriorate any other properties thereof.
Accordingly, there is a need to develop a composition of a non-aqueous electrolyte solution capable of providing a lithium battery having superior characteristics in terms of life cycle, low and high temperature discharging, as well as high-rate charging/discharging characteristic.